The present invention generally relates to a primary battery and, more particularly, to a silver oxide cell of flat type comprising a positive electrode composed of silver oxide.
With development of the electronics technology, most electronic instruments, such as electronic calculators and electronic wrist watches, are currently manufactured in smaller and smaller size and this trend has now increased a demand for compact and small batteries which may be used as an external source of electric power to be installed in these electronic instruments. A button-type dry cell is one of the smallest primary batteries now commercially available and is disclosed, for example, in the U.S. Pat. No. 3,615,858, patented on Oct. 25, 1971. However, unlike the button-type dry cell, being somewhat complicated in shape, a flat-type dry cell of a substantially disc-like shape is also commercially available, which flat-type dry cell will now be discussed with reference to FIG. 1 of the accompanying drawing since the present invention pertains thereto.
The prior art flat-type dry cell, specifically, the silver-zinc cell, shown in longitudinal sectional view in FIG. 1, has a two-part container comprising a cap 1 and a cup 6. The cap 1 is made of either a clad plate of stainless steel lined with copper or tin-plated nickel steel plate and serves as a negative terminal member. The cap 1 has its peripheral portion flanged at 1a in a direction substantially perpendicular to the plane of the remaining central portion 1b of the cap 1 and then bent outwardly backwards to assume a substantially U-sectioned anchor rim 1c.
This cap 1 houses therein a zinc negative electrode 2 composed of a compacted mixture of amalgamated zinc powder with sodium salt of carboxymethyl cellulose expanded by absorption of electrolyte.
The cup 6 is made of either nickel-plated steel or stainless steel and serves as a positive terminal member. This cup 6 has its peripheral portion flanged at 6a in a direction substantially perpendicular to the plane of the remaining central portion 6b of the cup 6 and in a direction towards the cap 1. The cup 6 houses therein a silver oxide positive electrode 5 composed of a mixture of silver oxide and powdery graphite compacted by the use of a binding agent such as polytetrafluoroethylene.
The cup 6 is electrically insulated from the cap 1, and vice versa, by means of an annular gasket 7 which is firmly held in position substantially between the flange 1c of the cap 1 and the flange 6a and annular portion, adjacent the flange 6a, of the central portion 6b of the cup 6 while the flange 6a is crimped inwards during assembly of the cell to substantially hermetically seal the latter. The annular gasket 7 is made of polyamide resin or any suitable resilient electrolyte-resistant material.
Positioned between the zinc negative electrode 2 and the silver oxide positive electrode 5 within the sealed two-part container is an electrolyte-absorbent layer 3, made of matted cotton fibers and situated adjacent and in contact with the negative electrode 2, and a semipermeable separator 4 made of a regenerated cellulose membrane, having its opposed surfaces coated with polyvinyl alcohol, and situated adjacent and in contact with the silver oxide positive electrode 5.
In practice, prior to the two-part container being substantially hermetically sealed in the manner as described above with a stack of the elements 2, 3, 4 and 5 housed therein, alkaline electrolyte is injected into the two-part container and, thereafter, the flange 6a is crimped inwards to substantially hermetically seal the container. The separator 4 is then held in position with its peripheral portion 4a firmly sandwiched between the annular gasket 7 and that annular portion of the central portion 6b of the cup 6 as shown.
In the prior art flat-type dry cell of the construction described above and shown in FIG. 1, the positive electrode 5 is in the form of a disc having a uniform thickness.
As is well known to those skilled in the art, any one of monovalent and divalent silver oxides has a strong oxidizing power and tends to be dissolved partially in the form of silver acidic ions, so that the separator tends to be deteriorated accompanying precipitation and penetration of metallic silver. The deterioration of the separator is considerable with elevation of the temperature and, therefore, depending on the type of application in which the prior art silver-zinc cell is employed, the separator is often employed in the form of an ion-exchange membrane which is generally considered as having an excellent resistance to oxidization.
It has been found that, when silver-zinc cells, each being of the construction shown in FIG. 1, were stored at 45.degree. C. for three months, about 35% of the total number show complete internal self-discharge and the rest have only 20 to 50% of the rated capacity. Close observation of the cells so tested and subsequently disassembled has shown that, while a central portion 4b of the separator 4, which is in contact with an upper face of the positive electrode 5, has not remarkably deteriorated, an annular portion adjacent the periphery of the separator 4 which is in contact with a substantially right-angled peripheral edge 5b on the upper face of the positive electrode 5 was considerably oxidized, and some have shown that the separator 4 had an annular area of breakage substantially in conformity with the contour of the peripheral edge 5b of the positive electrode 5. This phenomenon appears to have resulted from a set-up of internal stresses in the separator due to the sharp contact of the substantially right-angled peripheral edge 5b of the positive electrode 5 with the separator.